1. Field of the Invention
This invention relates generally to prosthetic implant devices for the spinal column, and more particularly to prosthetic devices for replacing unstable and/or surgically excised vertebral bodies of the thoracolumbar region of the spine, and methods for their insertion and fixation.
2. Related Art
The treatment of acute and chronic spinal cord injuries is rapidly advancing. Many new techniques designed to return function to an injured spinal cord are currently undergoing clinical studies. These new techniques generally require that the spinal cord be adequately decompressed. Typically, portions of the anterior vertebral column (i.e., the vertebral body) are removed to access the anterior portion of the spinal cord.
Anterior surgery is the preferred method for correcting vertebral body injuries for several reasons. First, the material causing the injury usually lies anteriorly and is therefore most easily removed from this aspect. Also, the motor tracks lie predominantly on the anterior aspect of the spinal cord and may more easily be restored from this approach. Finally, it is generally agreed that decompression of the neural tissue to facilitate a patient's recovery from acute traumatic spinal cord injuries is best accomplished through an anterior approach to the spine.
Unfortunately, the amount of anterior vertebral bone necessarily removed to expose the relevant section of the spinal cord or to achieve adequate decompression greatly increases the instability of the anterior and middle spinal columns. It is essential to the success of most spinal cord injury treatment techniques that the bony vertebral column be reconstructed to prevent instability or compression from compromising the neural repair.
Currently, the spinal column is typically reconstructed by grafting bone into the defect created by removal of anterior vertebral bone. Stabilizing instrumentation was then applied posteriorly through a second surgical procedure. The disadvantage of this method is the need for the second procedure from a different approach. Also, immediate post-operative stabilization of the spinal column is not possible using this technique since fusion of the grafted bone with the adjacent vertebrae requires three to six months. Patients undergoing spinal surgery were therefore typically required to restrict their movement and wear external spine stabilization devices post-operatively until the graft had adequately healed.
Alternatively, a number of surgical implants have been used to anteriorly stabilize the spinal column. Most of these stabilizing devices include a bridge member which spans the vertebral defect (i.e., the site of an injured vertebrae or the void where a vertebral body has been removed). The devices typically include lateral supports by which they are secured to the lateral surfaces of the neighboring vertebrae using bone screws.
Lateral attachment of the devices, however, places the bone screws in a perpendicular orientation to the major axis of flexion and extension of the spinal column. It has been discovered that in this orientation, the bone screws are prone to dislodgement during application of bending and torsional forces. Additionally, since the mechanical forces applied to the devices when the spine is moved are greatest at the lateral attachment points, the bone screws may fracture the vertebral bodies to which they are attached. Also, the supports with which the devices are secured in place extend beyond the confines of the spinal column. Impingement against adjacent visceral and vascular structures may therefore occur, potentially causing serious complications. Finally, the laterally applied devices still require the placement of a structural bone graft in the spinal column.
Other available prosthetic devices comprise turnbuckle-like articulating members secured to adjacent vertebral bodies with axial pins. These devices are installed by positioning them into the void left after a vertebral body has been removed and then turning the turnbuckle to drive the pins into the subjacent and suprajacent vertebrae. While these devices are fully embedded within the spinal column upon installation, a major disadvantage of these devices again lies in their instability. Specifically, these devices are prone to tilting upon application of the mechanical forces generated by spinal movements, resulting in eventual dislodgement. Furthermore, the articulating components of these devices are known to fail through fatigue and wear. For these reasons, these devices do not present a permanent stabilizing solution.
A need remains for an anterior vertebral body prosthetic device which provides immediate post-operative stabilization of the spinal column and which is also suitable for permanent implantation.